Calcareous Nodules (‘ Coal Balls’) and Their Significance. 255 
They contain petrified roots and rootlets. The common 
occurrence of such roots and rootlets in underclays supports 
the view that the latter represent old soils on which the Coal 
Measure forests grew. 
‘ Seam ’ nodules were originally thought to have been 
transported by water, their rounded form suggesting that they 
has been rolled as pebbles. Closer inspection, however, 
reveals an irregular surface, quite different from that of 
water-worn pebbles. In some cases concentric bands, similar 
to those in ordinary mineral concretions, can be seen in 
‘ floor ’ and ‘ seam ’ nodules. The rounded form of ‘ floor 
and ‘ seam ’ nodules is thus explained by the fact that they 
are concretions , formed around nuclei in the plant debris. There 
is no record of (transported) erratic blocks in the Upper Foot 
Coal which contains many 4 coal balls.’ Again, at the Shore and 
Wirral Collieries and elsewhere, the whole thickness of the seam 
has been locally converted into calcareous nodules. Finally, in 
some instances, a stem or root which is petrified in a 4 coal ball ’ 
can be traced into the surrounding coal (where it is coalified), 
while in other cases different parts of one and the same stem 
may be found petrified in neighbouring ‘ seam ' nodules. 
There is thus abundant evidence pointing to the conclusion 
that ‘ seam ’ nodules usually consist of members of plants 
which formed part of the seam itself, and that the debris 
usually accumulated in situ. 
Roof ’ nodules show distinct, though fine, bedding in the 
same direction as the bedding of the surrounding shales. The 
marine shells they contain ( Aviculopecten , goniatites, etc.) 
often occur in bands along the bedding planes. These facts, 
and their occurrence in the shaly roof of the seam, show that 
they were formed from drifted material. The roofs of many 
seams (not containing ‘ coal balls ’) contain non-marine shells 
(‘ mussels ’) indicating that the plant debris was submerged 
by an incursion of fresh — or (more usually) brackish — water. 
The fact that the roofs of 4 coal ball ’-containing seams include 
marine animals indicates that it was an incursion of the sea 
which submerged these particular coal forests. 
It was formerly thought that the petrifaction of plant 
tissues to form * coal balls ’ was due to the infiltration of 
minerals from the marine roofs of the seams. By experiment 
Stopes and Watson showed, however, that delicate plant 
tissues may be preserved temporarily from decomposition by 
the sulphates in sea water. From this it was inferred that 
the plants which formed 4 coal ball ’-containing seams grew on 
swamps with definitely brackish water round their roots (under 
conditions similar to those of the present-day Mangrove 
swamps). They are so-called maritime coals, the preserving 
and petrifying minerals (sulphates of lime and magnesia) 
1933 Nov. 1 
